The rotation direction of a magnetic island in the saturation regime and the underlying physical mechanism are numerically investigated based on a four-field model that includes the effects of both ion and electron diamagnetic drifts as well as parallel ion motion. It is found that diamagnetic effects vanish inside the island, and that the rotation direction is determined by nonlinearly generated zonal flow. The direction of zonal flow is sensitive to the viscosity and the finite Larmor radius (FLR) effect. The radial mode structure of zonal flow is found to be deformed by that of other modes as the viscosity increases. We have also shown that the FLR effect enhances island rotation toward the ion diamagnetic drift direction through energy transfer to the zonal flow by a nonlinear ion diamagnetic stress tensor.
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